1. Field of the Invention
The invention pertains to electronic color image acquisition, as well as digital image storage, retrieval and transmission.
2. Description of the Related Art
Digital images are normally captured in RGB color space, meaning that each pixel or point in the image is characterized by three values indicating the amount of red, green, and blue light present at that point. Typically, the filters used (either in a mosaic or in a sequential color system) partition the light energy into three bands according to wavelength, so the shortest wavelengths are recorded as blue, the middle range as green, and the longest range as red. Typically the blue range is 400-490 nm, green is 490-580 nm, and red is 590-660 nm. Then, the overall signal levels of red, green, and blue are adjusted to achieve a white balance. Often, color-correction matrix is applied to increase color separation, normally in the form of a non-diagonal 3xc3x973 matrix that is multiplied by the raw [R, G, B] data for each pixel (expressed as a column vector), to produce the improved [R, G, B] output data for that pixel.
There are problems with this approach. First, the definition of the three primaries is not universal amongst all manufacturers and all types of equipment. Second, and more fundamental, the color basis used to acquire images is not colorimetric in nature. This means it is possible to have two objects in a scene which appear to have a different visual hue or brightness from one another, yet an RGB camera would record the objects as having the same RGB reading. Conversely, one can have two objects which present the same visual appearance to the eye, yet which would be recorded as having different RGB color values.
The reason for this problem is that the color response of the camera weights different wavelengths of light differently from how the human eye does. As a result, the xe2x80x98rednessxe2x80x99 of an object recorded by the camera is not equivalent to the redness perceived by the eye, and similarly for other hues.
There is no way to retrieve the actual color from the digital image, once it has been captured in non-colorimetric terms. The loss of color information occurs at the time the image is recorded, due to the difference between the camera""s color weighting and the human eye""s color perception. No cameras or scanners in the prior art record the colors using the same color response that the human eye affords. As a result, color fidelity of digital images is lost at the time the image is captured.
The color error can be quantified for a given camera using measures such as the difference in L*a*b units between the color as recorded, and the true color of the object. A recent paper determined the relative spectral response of a Kodak DCS-200 and DCS420 camera, from which the color error may be determined for objects with various color spectra. In some cases, errors up to 20 L*a*b units are found, where 1 color unit represents the limit of human perceptibility.
Another problem occurs when one seeks to transform between an RGB color representation of a scene and other color representations such as cyan-yellow-magenta-black (CMYK) that is used in printing. This can lead to further degradation of color fidelity. Part of the reason for this is that the primaries in the other color representation space may not be well defined, i.e. the standard yellow, cyan, and magenta are not agreed-on. If there were agreement on this topic one might expect it would be possible to transform between various color representations by means of a simple 3xc3x973 linear algebraic matrix without color fidelity loss, using techniques known in the art of color science. However, the use of a transformation matrix presumes that the original color representation weighted the various color components in the same fashion that the human eye does, or in some linear algebraic transform of this fashion. Since the RGB color image is ambiguous in the sense that the camera or scanner used to record it has a color response that is not the same as the color perception response of the eye, the color error in the original color space can be increased when transforming to another color space.
Another problem relates to the fact that the color error of different cameras is not the same, and further depends on the hues being captured. If the transformation matrix is optimized for a certain range of colors or wavelengths, other colors will not be transformed well. Similarly, a transformation that works adequately for one camera (with its attendant color error properties) may not work well for another camera, which has a different set of color errors. This has led to a profusion of ad hoc methods to xe2x80x98calibratexe2x80x99 various cameras and scanners, as are evident in software packages such as Apple""s Color Sync, Agfa""s FotoTune.
There is a well-established field of colorimetry, described in standard texts such as MacAdam, Color Measurement, or Hunter and Harold, Measurement of Visual Appearance. It encompasses the specification of color, the human perception of hues and intensity, and the visual appearance of objects. However, there is at present no equipment or technique for the photographic or digital photographic recording of images in colorimetric terms. That is, while it is recognized that the principles of colorimetry should be applicable to every point in an image, the prior art equipment for measuring the colorimetric properties of light only records a single point at a time, or at most a line at a time. It would be possible, by adding scanning means and taking a sequence of line or point readings, to assemble a complete image with such equipment, but it is not practical except in a research laboratory environment. No practical system exists for recording an entire image in colorimetric terms, rapidly and with high spatial resolution.
Related to this, there are various measurement tools, including calorimeters, spectrometers, and the like, which are used to check the color of printed materials, and the appearance of luminous displays such as cathode-ray tubes (CRTs). Some of these devices are placed near or in contact with a CRT display, and its color is read by computer while various color signals are put to it. In this way, the color distortions and other properties of the display are learned and that information is used by color management software to correct for deficiencies in the display. Similar technology exists for printers, LCD displays and other graphic output devices. However, there is no quantitative basis for insuring end-to-end color management unless both the acquisition and display alike are put on a quantitative basis and given high fidelity. The present practice may be termed an open-loop approach, with control over only a portion of the process.
CRI (Boston, Mass.) makes a tunable filter termed the xe2x80x98VariSPECxe2x80x99 which enables one to acquire an image at any specified wavelength. By using this filter to take many images that span the visible spectrum, multiplying the pixel intensity values of each image by the numerical value of the X colorimetric weighting function, and summing the reading of all images at each pixel, one can obtain the exact colorimetric value for the X response at each point in the image. The weighting and summing may then be repeated to obtain the Y and Z colorimetric values, at which point one has a high-resolution image of the scene with colorimetric color rendering.
Gemex (Mequon, Wis.) has made and marketed a gem-grading system which uses this approach to quantify the color of valuable gems, and to produce colorimetric-quality images. However, many exposures are required, typically 20 or more, from which the spectral data is extracted. The amount of data required the computing burden, and the time involvedxe2x80x94approximately one minute per complete imagexe2x80x94render this impractical for most uses.
Koehler, in U.S. Pat. No. 5,142,414 teaches a micro-mechanical etalon which purports to produce an electrically-variable optical transmission or reflection response. By altering the drive signal to an array of micro-mechanical actuators, one varies the spacing between members of an etalon, or places them essentially in contact. This is said to provide means for producing transmission functions that mimic the XYZ colorimetric weighting functions. However, the mechanism by which this is achieved is not clear, since the X, Y, and Z curves have quite different finesse from one another, and the X value is doubly-peaked within a given order. If such a system were constructed, it is believed the colorimetric matching would be of a poor quality.
There is at present no practical system of digital photography that preserves the colors accurately and with high fidelity. Nor does the present art provide for specifying the colors in digital photography in a way which can be objectively measured, and compared against standards at any point from acquisition through reproduction and printing. Thus despite the electronic photography revolution, the field of printing and reprographics remains an artisan trade, and customers of printing and graphic design cannot be sure that objects will be recorded and reproduced as they want. The increasing reliance on the Internet for commerce has led to a greater desire for true color in order that color decisions can be made with confidence when shopping.
It is the aim of this invention to provide a method and means for digital recording of a scene in colorimetric terms with comparable ease and speed to other methods of digital photography. It is another aim of this invention to obtain such images in XYZ space, L*a*b space and other colorimetric spaces that make efficient use of computer storage and digital transmission bandwidth.
One embodiment uses liquid crystal filters in a time-sequential approach to color imaging. The liquid crystal filters produce transmission curves that match the X,Y, and Z response functions divided by the spectral response of the electronic image detector. Thus, the overall system captures the X,Y, and Z components of every point in the scene in a digital photographic image. A variation of this further incorporates means to record the color properties of the ambient light.
Another embodiment of special use in medical imaging uses a spectrally-variable illuminator to illuminate the scene, which is then photographed at three illuminator settings. The illuminator settings are chosen to be the spectrum of a desired illuminant, times the X, Y, and Z response, divided by the spectral response of the detector. Thus, the overall system captures the X, Y, and Z components of the scene in a digital photographic image taken under controlled illumination conditions such as daylight or CIE C, CIE A, or the like.
These produce images that directly and quantitatively capture the color in colorimetric units at every point in the scene. The preferred embodiment further incorporates means for converting the raw images into XYZ space, taking account of overall signal levels and the relative exposure times employed. In some embodiments, there is included a further means for transforming from XYZ colorimetric space to an L*a*b space, which has the property that equal distances correspond to equal perceptual differences, so digital storage of the pixel appearance is efficient and compact for storage and transmission purposes.
This invention enables one to obtain images with greatly reduced color errors, or simply put, to take digital photographs with markedly better color quality. This will be an immediate benefit to digital photography users. Some embodiments incorporate means for transforming the image from XYZ colorimetric space to RGB space in a quantitative fashion that permits transformation back to XYZ or other colorimetric spaces without loss or degradation of color information.
A key long-term benefit of the Invention is that, since it records images in a quantitative fashion, it is possible at all subsequent steps to refer to the primary colorimetric data and know what colors were present. So, while a given monitor, printer, or display may have inherent color limitations and errors which prevent the perfect rendering of the image, one may utilize this Invention to preserve a true record of the colors as they ought to be present in the image. Presently, since a camera""s errors are comparable to those of any other component, it is normal practice that the color balance is altered by eye at each step: acquisition, layout, proofing, color separation, and printing. The resulting accumulation of errors, and the absence of any quantitative objective basis for end-to-end color checking, is eliminated by the present invention. It may be employed (in one or more embodiments) both to acquire the image and to confirm that it has been accurately reproduced. In both cases, the operation is accurate, quantitative and colorimetricxe2x80x94it responds in the same fashion as the human eye. Thus, end-to-end color management is realized.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.